The font for this test case attempts to run a denial-of-service attack on the rendering engine under test. The font’s finite-state transducer substitutes glyph A by B, B by C, C by D, D by E, and E by A. These transitions are all marked with the DontAdvance flag, so the machine never makes progress.

If your rendering system is immune to this attack, it will detect getting stuck (for example, by imposing an upper limit on the number of transitions taken by the finite-state machine), and give up quickly. See fontkit bug 175.